As is commonly observed, a person that has learned to speak a second (or third, etc.) language will typically have an accent. A significant aspect of the accent is in the formation of vowel sounds.
FIG. 1 is a graph illustrating an exemplary vowel sound in the frequency domain (i.e., magnitude as a function of frequency). The vowel sound, when plotted as the magnitude of the sound as a function of frequency, generally forms a curve 100 having a series of local maxima at various frequencies along the frequency band of interest, generally about zero to about five kHz. FIG. 1 shows four local maxima 101-104. The frequencies of the local maxima are commonly referred to as "formants". The formant with the lowest frequency is referred to as formant F1. Similarly, the second lowest frequency formant is referred to as formant F2, the third lowest frequency formant is referred to as formant F3 and so on.
A vowel sound may be sufficiently identified by the frequencies of formants F1 and F2. FIG. 2 is a graph illustrating an exemplary group of vowel sounds produced by a native speaker of English. In particular, the graph was formed by plotting, for each vowel sound in the language's vowel set, formant F2 of that vowel sound as a function of that vowel sound's F1 formant. For example, the vowel sound [i] (using the well-known IPA nomenclature) has a relatively high F2 formant with a relatively low F1 formant. Conversely, the vowel sound "a" has a relatively low F2 formant with a relatively high F1 formant. A monotonic curve 200 can be drawn containing the vowel sounds [i], [I], [.epsilon.], [.ae butted.] and [a], as shown in FIG. 2.
However, it has been observed that the vowel set of a person's native language may not include one or more vowel sounds in the new language that the person is speaking. It is thought that the person speaking is not sensitive to these vowel sounds in either hearing or speaking these vowel sounds. Thus, the speaker typically cannot determine whether he or she is correctly producing the target sound. Of course, this deficiency hinders the speaker's training in reducing the speaker's accent. Accordingly, there is a need for a system that helps speakers to become sensitive to target vowel sounds when speaking a non-native language.